1. Field of the Invention
Stabilization of metals in aqueous hazardous waste usually is accomplished by converting the metal ions into insoluble solid precipitate (i.e., hydroxide, carbonate, sulfate, etc.). When this type of precipitate is mixed in cement and the final product passes the leaching test, the stabilized waste can be placed in the appropriate landfill. In most cases, mixing the aqueous metal solution with a base (i.e., lime) and cement will suffice to stabilize the metals in the waste.
However, there are cases in which metals cannot be converted into the insoluble hydroxide, carbonate or sulfate because the metal is in an improper oxidation state. This causes the metal to leach from the "stabilized" waste during the leaching test. In some cases, this situation is corrected by changing the oxidation state of the metal prior to stabilization.
Metals held very strongly in a form of a chelate (i.e., EDTA) or a complex (i.e., cyanide) also fail the leaching test after "stabilization." Metals such as nickel, cadmium and cobalt for example cannot be converted into the insoluble form by a simple adjustment of pH or the action of anions such as sulfate or carbonate in such an environment. They are held too tightly by the complexing or chelating agent and cannot be precipitated. The best way to free the metal is to destroy the complexing power of the complexing or chelating agent. Since the complexing or chelating agent is an organic moiety, its destruction is usually achieved by oxidation of this organic material by incineration or hypochlorite oxidation. There are cases, expecially if the complexes are very resistant to chemical oxidation and/or are present in a dilute water solution, where the above mentioned treatment will not be able to convert the metals to a form that will be amenable for efficient stabilization. Therefore, a method to efficiently destroy the metal complexes in such cases is needed.
2. Prior Art
U.S. Pat. No. 3,920,547 describes a process for the destruction of cyanides in an aqueous solution. In the process, an aqueous cyanide solution is contacted with an ozone containing gas while simultaneously being irradiated with ultraviolet light. The pH of the aqueous cyanide solution is held within a range of 5 to 9.
U.S. Pat. No. 4,012,321 describes a method for treating refractory organic compounds. The aqueous waste stream containing organic material, exhibiting a chemical oxygen demand or biological oxygen demand, is mixed with hydrogen peroxide. The hydrogen peroxide containing organic solution is irradiated with ultraviolet light to destroy the refractory organic compound, thereby reducing the chemical oxygen demand of the waste stream.
U.S. Pat. No. 4,289,549 describes a process for treating a liquid waste stream containing complex copper ions. The copper ion concentration of the solution is first adjusted to about 8 ppm after which the solution is contacted with an ozone containing gas and irradiated with ultraviolet light.
U.S. Pat. No. 4,446,029 describes a process for the destruction of cyanide in an aqueous solution containing an iron cyanide complex. The iron cyanide complex is destroyed by adjusting the pH of the solution to 11 to 12 and irradiating the solution with ultraviolet light. Afterwards, the pH of the solution is reduced to between 8 and 11 and allowed to react with hydrogen peroxide.
U.S. Pat. No. 4,512,900 describes a process for treating liquid waste compositions containing copper ions and a complexing agent. The concentration of copper ions in the waste material is reduced to less than about 8 ppm. The copper ion solution is then contacted with hydrogen peroxide followed by an ozone containing gas. The waste composition is then irradiated with ultraviolet light. In the process, the hydrogen peroxide destroys from about 20 to 60 weight percent of the total organic content of the waste stream and the ozone containing gas reacts with and destroys the complexing agent.
U.S. Pat. Nos. 4,792,407 and 4,849,114 describe a method of oxidizing toxic compounds in aqueous solutions. The method consists of simultaneously exposing an aqueous solution containing toxic hydrocarbon compounds to ozone, hydrogen peroxide and ultraviolet radiation in amounts sufficient to reduce the toxic constituents of the solution. The method is particularly useful for reducing the amount of halogenated hydrocarbon constituents and partially halogenated hydrocarbon constituents in an aqueous solution.
U.S. Pat. No. 4,846,978 describes a process for decomposing metal chelates by exposing a metal chelate containing aqueous solution to ozone gas in the presence of magnesium hydroxide.
EPRI Report No. NP-4954, pp. 3-41 to 3-49 (Nov. 1986) discussed "oxyphotolysis" of various metal chelates. The report indicates that the destruction of metal-EDTA chelates was not accelerated upon exposure to UV radiation during ozonation.